Take a look at unfold-more-horizontal and unfold-more-vertical.unfold-more-horizontal is actually pointing in vertical direction and unfold-more-vertical is pointing in horizontal direction.
Aren't they supposed to have their names switched?
It's probably in the way it's interpreted. Unfolding a horizontal item is how I would view this.

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It's probably in the way it's interpreted. Unfolding a horizontal item is how I would view this.